dma-mapping: benchmark: extract a common header file for map_benchmark definition

kernel/dma/map_benchmark.c and selftests/dma/dma_map_benchmark.c
have duplicate map_benchmark definitions, which tends to lead to
inconsistent changes to map_benchmark on both sides, extract a
common header file to avoid this problem.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Acked-by: Barry Song <song.bao.hua@hisilicon.com>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>